Electrolytic fluid treatment systems are widely used to, for example, remove impurities and contaminants from fluids. In such systems, the fluid to be treated is passed between one or more pairs of electrodes. An electric potential applied to the electrodes establishes an electric current between the electrodes. As a result, impurities in the fluid migrate and adhere to the electrodes, biological materials in the fluid are killed, and the fluid's chemical composition may be altered.
One fluid that is commonly processed by electrolytic fluid treatment systems is water. The electrolytic treatment of water is, however, complicated by the widely varying water characteristics encountered from one water source to another. In that regard, the resistivity of water, which is inversely proportional to conductivity, commonly varies over a range extending from 30 to 1400 ohm-meter. Such resistivity variations may significantly alter the performance of an electrolytic filter system.
More particularly, the interelectrode resistance is dependent upon the resistivity of the water flowing between the electrodes. With a fixed electric potential applied to the electrodes, current flow between the electrodes will vary in inverse proportion to the water's resistance. If water resistivity is relatively high, the current may be too low to achieve the desired treatment of the water. On the other hand, if water resistivity is relatively low, the current may be so high as to damage or otherwise decrease the life of system components.
A variety of different systems have been developed that attempt to accommodate such variations in water resistivity. For example, electronic control circuits have been designed to allow water purification and ion generation systems to maintain constant current flows, substantially independent of variations in water resistivity.
In that regard, U.S. Pat. No. 4,119,520 (Paschakarnis et al.) discloses a water purification unit that includes such a current control circuit. The current to be controlled flows through a resistor, as well as between the electrodes. A differential amplifier and transistor cooperatively control the current by keeping the voltage drop across the resistor equal to the reference potential across a diode. As a result, the current flowing between the electrodes is kept constant.
Similarly, U.S. Pat. No. 5,055,170 (Saito) discloses an ionic water generator that accounts for variations in water resistivity. The Saito system employs a central processing unit that calculates the appropriate voltage to be applied to the electrodes for the water being processed. This voltage is computed by multiplying some voltage corresponding to the desired ion concentration by a factor equal to the resistance of the water actually being processed divided by the resistance of some reference water.
As will be appreciated, the Paschakarnis et al. and Saito systems exhibit several shortcomings. First, the control circuits of both systems are relatively complex. Because the Paschakarnis et al. circuit introduces an additional resistance into the current path, it is also relatively inefficient. The Saito circuit, in turn, disadvantageously requires reference measurements to be made for subsequent use in controlling the voltage applied to the electrodes.
An alternative method of handling variations in water conductivity is to ensure that the system is exposed to a relatively constant load resistance, regardless of variations in water resistivity. U.S. Pat. No. 4,769,119 (Grundler) discloses a water ionizing device, including several electrodes, that employs this approach. If the resistivity of the water being ionized is relatively low, a relatively high resistance is introduced in series with the electrodes. On the other hand, if the water's resistivity is relatively high, a relatively low resistance is introduced in series with the electrodes. In either case, by keeping the system's total resistive load constant, a constant current flow is maintained between the electrodes.
U.S. Pat. No. 4,986,906 (Dadisman) describes another variation of this approach. The Dadisman water purification system includes a constant current control circuit in which changes in water resistance cause opposing changes in the effective resistance of a field-effect transistor (FET) included in the circuit. These changes in FET resistance offset the changes in water resistance, allowing the current to be kept substantially constant.
The Grundler and Dadisman systems also have certain limitations. More particularly, the Grundler and Dadisman circuits both increase circuit resistance to offset decreases in water resistance. As a result, energy is dissipated in circuit components rather than being used to filter water, making the circuit relatively inefficient. In addition, the Grundler and Dadisman circuits are also both relatively complex.
Yet another technique proposed to handle variations in water resistivity is described in U.S. Pat. No. 3,691,050 (Sayre). The Sayre water treatment cell includes electrodes whose separation is mechanically adjustable. If the cell is to be used with water having a relatively high resistivity, the operator physically adjusts the electrodes so that they are closer together. Alternatively, if the system is to be used with water having a relatively low resistivity, the operator adjusts the electrodes so that they are more widely spaced. In either event, the interelectrode resistance is kept uniform, ensuring a constant current flow.
As will be appreciated, the Sayre system has several shortcomings. First, the operator is required to make independent assessments of the water's resistivity. In addition, the system is relatively complicated and the necessary adjustments are relatively time consuming to perform. Finally, because the system does not automatically respond to variations in water resistivity, it may fail to achieve the desired regulation in many instances.
As will be appreciated from the foregoing remarks, it would be desirable to provide a electrolytic filter system that is substantially free from the influence of water resistivity variations, while remaining relatively efficient, simple, and easy to operate.